Price to fill up (6)
I believe the answer is:
charge
This clue is a double definition.
'price' is the first definition.
(I've seen this before)
'fill up' is the second definition.
(charging is a kind of filling up)
'to' acts as a link.
(Other definitions for charge that I've seen before include "Formally accuse (someone)" , "money to be paid" , "Keeping" , "impeachment" , "Price for a service" .)